Conventional cradle-mounted navigation devices having an inertial navigation system (INS) become disoriented when the navigation device is moved relative to the vehicle to which the cradle is mounted, if the navigation device uses its fixed orientation with respect to the vehicle as a constraint. The constraint is usually applied to enhance the navigation solution. When the cradle is rotated and/or moved relative to the vehicle, a mathematical alignment of the INS to the vehicle coordinate system breaks, resulting in an incorrect navigation solution. For example, a passenger may rotate a cradle holding the navigation device to view the device's screen, thus changing the device's position and orientation relative to the vehicle. The navigation device's INS improperly measures the passenger's movement of the navigation device as a differential movement of the vehicle. Moving the cradle induces gross errors into the conventional navigation device's calculations.
In other words, to provide good navigation results, the conventional navigation device that is cradle-mounted in a vehicle needs to be mathematically aligned to the vehicle coordinate system in order to take advantage of an algorithm accounting for a non-holonomic constraint (NHC). An NHC causes a state of an object to depend on a path taken by that object in achieving the state. The NHC prevents movement in at least one direction, and the NHC exists when a number of degrees of freedom in a path exceeds the number of independent motions that the object taking the path can produce. For example, a bicycle cannot be pedaled sideways from a first location directly to a second location that is parallel with the first location. The constraint that the bicycle cannot be peddled sideways, even though the rider must move to the second location, is an NHC. It is noteworthy that the NHC limits a direction in which the object can move, but does not limit the locations to which the object can move. Continuing with the example above, the NHC does not prevent the bicycle rider from moving to the second location, because the bicycle can be pedaled in a circular direction into the second location.
Accordingly, there are long-felt industry needs for methods and apparatus that mitigate the shortcomings of conventional methods and apparatus, including an apparatus and method for detecting and measuring cradle movement, as well as an apparatus and method for mitigating inertial navigation errors induced by cradle movement.